It's not the mists of time that lay claim to abandoned cities. It's the undergrowth.

Fast-spreading vines and other weeds are among the first tentacles Mother Nature sends up to grip a deserted city. So it was that Troy, Chichen Itza, Angkor and other metropolises of antiquity vanished and were forgotten. Other classical cities, including Rome, once became partially lost to greenery and decay.

Now, in hurricane-ravaged and largely abandoned parts of New Orleans, the timeless process is being replayed. In swathes of the once-submerged Lower Ninth Ward, for example, houses, trailers and sidewalks lie neglected and disappearing. The weeds appear to be taking over.

The rapidity of nature's resurgence in the fertile Crescent City is no surprise to scientists. "We've got exuberant vegetation down here in Louisiana," says Steven Darwin, a botanist at the city's Tulane University. Before Hurricane Katrina in August 2005 and the attending floods, he says, "people were mowing their lawns, keeping this vegetation under control."

But since then, plenty of spots have gone to seed. "My neighbors haven't been back for two years," says Darwin, whose uptown neighborhood, Carrollton, didn't get flooded. "You should see their backyard. It's almost impenetrable."

Mike Davis, 19, echoed that sentiment as he surveyed the yard where he grew up on Tonti Street in the Lower Ninth. He and his brother Jason, 18, still have relatives in the neighborhood but have resettled across the Mississippi in Harvey, La. During a return visit, the two sat on the hood of their car in front of their old home, which lies half -obscured behind trees and weeds. "There's no more house," Mike Davis says. "It's a forest."

He may have been overstating the case — or he may have been foretelling the future. "In a few decades, a lot of this stuff will just be rubble on the ground," says Alan Weisman, author of The World Without Us, which describes how evidence of civilization would gradually evaporate if humanity vanished.

"Cities can be buried extraordinarily quickly," says C. Brian Rose, president of the Archaeological Institute of America. Typically, he explains, a catastrophe — armed conflict, for example, or an earthquake, fire or flood — will first depopulate a city. Then seeds will blow in on the wind, take root and grow, particularly if a flood has deposited a new layer of soil.

Weeds also may sprout from buried roots, which can be surprisingly resistant to floodwaters. And if remaining residents don't put brakes on the process, the city's architecture is doomed. "Roots will grab hold of the foundations and bring down the structures over time, even if they're stone," Rose says. "In time, nothing will be left. All you'll see is foliage."

It isn't clear what plants would make up such a blanket of flora in the Big Easy, but it would be unlikely to include grasses or garden plants and shrubs. To survive, such ornamentals require constant human attention, and, in any case, the salinity of the floodwaters might have done them in. Though some old New Orleans species, including live oaks, fared reasonably well against the brackish flood, magnolias and other trees suffered heavy losses, experts say.

"Vines are really successful in these situations," says Allison Snow, a plant ecologist at Ohio State University. "They can grow really fast because they rely on other things to hold them up" — other things like houses, lampposts and street signs.

Invasive species also tend to become dominant in disturbed ecosystems, says Conrad Savy, a biodiversity analyst at Conservation International. Tulane ecologist Jeffrey Chambers says one such invader — the Chinese tallow tree — already has made rapid inroads.

"Open areas (will) eventually revert into some kind of a forested ecosystem," Chambers says. Darwin agrees and adds: "But it's not going to be the 'right' kind of forest. It's not the seeds of the native tree species that will come in. It may well be the seeds of the exotics that are lurking in the woodlands, especially Chinese tallow."

Though the emerging ecosystem may be unfamiliar, it's no less natural, Weisman says. "These plants are just doing what comes naturally to them. They've been given an opening, and they're going for it."

To turn them back, and to avert the post-disaster cycle of urban decay and abandonment, will require Herculean effort, experts say. "The chances of reclamation decline with every building that falls down," says Rose, who has spent two decades sifting through the ruins of Troy. Yet people who remain "may decide to intervene and, through sweat equity, bring it back."

Volunteers for the group Common Ground seem willing to sweat. In the Lower Ninth Ward, work crews often start bushwhacking by 8 a.m., says volunteer coordinator John Freeman.

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